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Brief Synopsis

As he admires his latest art acquisition with a friend, rich, old Marsden is murdered by the notorious killer Black Ace, so called because he always leaves an ace of spades playing card on his victims' bodies. Shortly afterward, crime story writer Neil Broderick, who is on his way to Chicago to meet Thornton Drake, a wealthy friend of Marsden's who has been following the investigation, encounters Martha Winters, the daughter of Drake's secretary, Austin Winters. Immediately after Neil's arrival at Drake's mansion, an agitated Winters fits together the final pieces of a jigsaw puzzle that had been sent to Drake, which form a picture of a black ace with the words "at seven tomorrow night" embossed on it. To protect himself, Drake flies to his Louisiana plantation the next day with Neil, the Winters and Chicago detectives Clancy and Dugan. In mid-air at seven o'clock, the plane lights suddenly go out, and when they finally are restored, Winters is dead. Convinced that the murderer was on the airplane, bumbling, cowardly Clancy and Dugan hold and question all of the passengers and the pilot at the plantation, while Neil undertakes his own clandestine investigation. Late that night, a coroner who has been summoned by Neil, finds a letter on Winters' body, which Drake begins to read aloud. Before he can finish the letter, in which Winters claims to know the identity of the Black Ace, the lights go out again and the letter disappears. Neil determines that Martha stole the letter to protect her father's name, but by the time they return to fetch it from its hiding place, only blank paper and a black ace are found. After the pilot is murdered and the coroner is revealed to be an F.B.I. agent, Neil, who has become the inept detectives' prime suspect, accuses Drake of the killings, and a fierce fight ensues. Finally, Drake dies by his own weapon, and the letter is restored, thus proving Neil's innocence to Martha and the bemused Clancy and Dugan.